With the increasing miniaturization of integrated circuits and the development of consumer electronic devices such as mobile phones, notebook and laptop computers, personal digital assistants, and digital cameras, the obvious market trend is toward making these devices lighter, thinner, and more compact. Thus, various electronic components should be manufactured and integrated into consumer electronic devices in such a way as to take up less space, yet provide more functions and improved performance. In the cell phone industry, smartphones in particular need to integrate various electronic components having a small volume, multi-functionality and low-cost properties in a specific volume, for example, integrating transmission mediums such as microphones with other communication devices.
Electric condenser microphones (hereinafter referred to as ECM), which are constructed using electret materials, are the most-often used microphones in consumer electronic devices. However, the ECM has been gradually replaced with a micro-electro-mechanical system acoustic transducer (hereinafter referred to as MEMS acoustic transducer). In general, both the ECM and the MEMS acoustic transducers detect sound by sensing the capacitance variation produced by acoustic pressure. In the ECM, a capacitor is formed of electret polymer membranes having eternal isolated charges for sensing the capacitance variation. In the MEMS acoustic transducer, there is a MEMS chip and an ASIC chip. The MEMS chip includes a capacitor formed of a membrane and a rigid through-hole back electrode on a silicon substrate for sensing the capacitance variation from the acoustic pressure, and the capacitance variation is processed by the ASIC chip. When comparing the ECM with the MEMS acoustic transducer, the latter may have a lot of advantages such as low cost, at least 30% of height reduction in its packaging structure, and resistance to degradation due to temperature, moisture, vibration, and general wear and tear. Moreover, the MEMS acoustic transducer is capable of being integrated with a band RF filter on ICs to reduce the interference produced by the RF, and the noise can be eliminated using arrays and algorithms. Thus, the MEMS acoustic transducer is especially suitable for RF applications such as cell phones and other devices that operate along similar principles, such as hearing aids, for example.
Thus, it is predicted that the MEMS acoustic transducer will largely replace the electret condenser microphones as the related technology continues to improve. Sensitivity is a key indicator of the MEMS acoustic transducer's effectiveness. Sensitivity is not only determined by a membrane in the MEMS chip, but is also determined by the volume of a back cavity. The volume of the back cavity is a closed volume behind the membrane and stands in contrast to the encountered acoustic pressure, which may provide a flexible recovery force to the membrane and can be used for tuning the acoustic resistive and response properties of the MEMS acoustic transducer. In addition, the fabrication process of the MEMS acoustic transducer is complicated, and therefore it is difficult to increase the sensitivity of the MEMS acoustic transducer while reducing production costs.
FIG. 1 shows a MEMS acoustic transducer packaging structure. In order to increase the volume of the back cavity 107 in a limited space, an interior housing 111 is added. The MEMS acoustic transducer includes a cavity 106 surrounded by a packaging substrate 102 and a housing 104. The housing 104 has a sound-opening 112 for receiving acoustic pressure. A MEMS acoustic transducer 116 and an application-specific integrated circuit (ASIC) chip 126 are disposed on the interior housing 111 within the cavity 106. The MEMS acoustic transducer 116 includes a silicon substrate 120, a membrane 118 (upper electrode), and a backplate (through-hole back electrode) 114 suspended below the membrane 118. The interior housing 111 and the packaging substrate 102 create the back cavity 107 of the MEMS acoustic transducer 116. Thus, the height of the back cavity 107 is similar to that of the MEMS acoustic transducer packaging structure when excluding of the height of the MEMS acoustic transducer 116, and therefore the volume of the back cavity 107 can be increased. However, there is still some space inside the packaging structure that cannot be used efficiently. In addition, the membrane 118 and the backplate 114 of the MEMS acoustic transducer 116 are two thin films which are hard to fabricate and easy to stick to each other. Thus, the MEMS acoustic transducer, as shown in FIG. 1, still cannot meet requirements of future applications.